1. Field of the Invention
The present invention relates to a semiconductor device that exhibits an increased resistance against radiation-induced malfunctions, and, more particularly, to a semiconductor device having a reduced penetration rate of alpha particles.
2. Description of the Related Art
Steadily decreasing feature sizes in modern integrated circuits allow fabrication of electronic devices exhibiting complex functionality within an extremely small volume. Accordingly, modern integrated circuits are increasingly used in all types of electronic devices as data processing units or as storage media, irrespective of whether the device is an everyday product, such as a personal computer, or a device employed in the medical, technical or scientific fields. Among this variety of possible applications of integrated circuits, certain critical applications, e.g., data processing units in vehicles, medical devices and the like, require extremely reliable semiconductor devices, such as chips with logic and/or memory function, to avoid serious malfunction of the semiconductor device and any peripheral devices connected thereto. Due to the ever decreasing feature sizes of modern integrated circuits, however, radiation-induced charge carrier generation in semiconductor devices increasingly proves to be a possible source of failure for the device, which accordingly decreases reliability, or even causes a complete failure, and thus restricts applicability of the device.
The issue of radiation-induced charge carrier generation becomes more exacerbated with decreasing supply voltage driving modern integrated circuits that are preferably used in portable devices. High energetic particles penetrating the semiconductor device may create a plurality of electron hole pairs, which may then enter charge-sensitive areas and cause device failures, and/or may accumulate in charge-sensitive regions of the semiconductor device, for example in dielectrics of storage capacitors or in gate insulation layers of MOS transistors. In the latter case, a significant drift in the threshold voltage of the transistor device may be created, whereas an accumulated charge in a storage capacitor may lead to a changed logic state of the capacitor, thereby causing an error in reading the storage contents, which is also referred to as soft error.
It has been found that a major source of radiation-induced charge carrier generation is the emission of alpha particles from materials of which the semiconductor devices are formed or which are used for assembly and packaging. In particular, semiconductor devices that are packaged by using a plurality of solder bumps may in general contain a large fraction of lead. Lead as usually used for standard solder materials contains the isotope 210Pb that undergoes a β decay and forms an unstable 210Bi isotope and a Po isotope, which then decay into a stable 206Pb, thereby emitting alpha particles with an energy of about 5.4 MeV. Alpha particles that are moving upon decay of a lead isotope 210 in the direction of charge-sensitive areas of the semiconductor device create an especially large amount of additional charge carriers due to the large absorption cross-section of alpha particles in matter.
U.S. Pat. No. 5,965,945 addresses the problem of alpha particles inherently created by the decay of 210Pb isotopes contained in the solder bumps and proposes an improved solder bump composition in which a thin low alpha layer of lead is deposited over alpha particle sensitive devices, while ordinary, i.e., low cost, lead is used for the bulk of the solder bump. Although this approach allows effective absorption of alpha particles emitted by the low cost lead, the provision of low alpha lead is costly and it is difficult during the reflowing of the solder bump for forming a solder ball to avoid mixing of the low alpha lead and the low cost lead.
U.S. Pat. No. 6,043,429 discloses a flip chip and a flip chip package that are shielded from alpha particles in that the solder bumps are coated with a layer of alpha particle absorbing material or in that a suitable amount of alpha particle absorbing material is provided in the underfill material between the flip chip and the package substrate. In this arrangement, the penetration of alpha particles is significantly reduced, wherein, however, the penetration of alpha particles, inherently created by the decay of alpha active isotopes that are moving directly from the solder bump into the underlying device regions, may not be stopped efficiently.
With reference to FIG. 1, the problem of penetration of high energy particles into charge-carrier sensitive regions will now be discussed in more detail, wherein a typical prior art semiconductor device including, for instance, MOS transistors or storage capacitors, is described.
In FIG. 1, a semiconductor device 100 comprises a substrate 101 that includes one or more functional elements (not shown) that are sensitive for radiation-induced charge carriers. A contact pad 102 is formed over the substrate 101 and is usually in electrical contact with the functional element. The contact pad 102 is electrically insulated by a first insulating layer 103 and a second insulating layer 104. On the contact pad 102 and partially on the second insulating layer 104, a metal or metal compound layer, also referred to as underbump metallization 105, is formed and separates a solder ball 106 comprising a substantial amount of lead from the underlying material layers.
Process flows for forming the semiconductor 100 are well known in the art and a detailed description thereof is omitted. It should be noted, however, that the solder ball 106 is formed of a solder bump that may be deposited over the underbump metallization 105 and partially over the second insulating layer 104 by means of electroplating or any other appropriate deposition method using a mask to adequately dimension the solder bump. After removal of the mask, the solder bump is reflowed to form the solder ball 106 which substantially recedes onto the underbump metallization 105 due to surface tension. The underbump metallization 105 substantially serves two purposes. First, the underbump metallization 105 is provided to substantially prevent diffusion of solder material into the underlying regions of the semiconductor device 100. Second, the underbump metallization 105 has to provide sufficient adhesion to the materials over and under the underbump metallization to establish a required mechanical stability and to guarantee the required reliability.
During usage of the semiconductor device 100, unstable lead isotopes, such as the isotope 210Pb, may decay and, as a result of this decay, alpha particles may be generated. As an example, in FIG. 1, one branch of the decay of the 210Pb isotope is depicted. If the alpha particles are generated sufficiently close to the interface of the underlying material layers, such as the underbump metallization 105, the alpha particle may also penetrate the substrate 101 and create a plurality of electron hole pairs until the alpha particle is finally stopped. As previously explained, a fraction of these additionally created charge carriers may enter charge-sensitive areas, such as junctions between inversely doped regions, or thin dielectric layers separating electrically active regions. Consequently, this additional charge may cause a significant shift of the operating conditions, especially when the feature sizes are small and the corresponding operating voltages are low.
In view of the problems outlined above, there is a need for an improved semiconductor device in which penetration of high-energy particles, especially alpha particles, is significantly reduced.